Fix/missing template header

[ryanwelcher]

What?

Follow up to #49345 to add a missing header to the template

[github-actions]

Flaky tests detected in b35e97a.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/4513939049
📝 Reported issues:

• [Flaky Test] can create template parts via the block placeholder start blank option #49016 in /test/e2e/specs/site-editor/template-part.spec.js